1. Field of the Invention
This invention relates to hydrophobizing silica particles, including colloidal silica, and, more particularly, to hydrophobizing silica particles with hydrosiloxane agents.
2. Description of the Related Art
Electrophotographic image formation comprises developing an electrostatic latent image formed on a photoreceptor with a toner comprising a colorant dispersed in a binder resin, transferring the toner image to a receiving paper, and fixing the transferred toner image by means of, for example, a hot roll. The photoreceptor is cleaned after the transferring step to prepare for the next latent image formation.
Silica particles are often combined with toner particles in order to improve selected properties of the toner particles, including fluidity, transferability, fixability, and cleaning properties. Typically, the silica particles are subjected to a chemical treatment to render the surface of the particles hydrophobic. Many different methods are known for treating the surface of silica particles.
Traditionally, silica particles have been treated with a variety of agents, for example, polydimethylsiloxane (PDMS), in fluidized bed reactors at elevated temperatures, e.g, 250-300° C. At these elevated temperatures, two processes take place simultaneously: (i) depolymerization of the polymer into smaller volatile fragments (e.g., D3 and D4) that evaporate from the system and (ii) oxidation of methyl groups, breaking the methyl-silicon bond (see Dvornic, “Thermal Properties of Polysiloxanes,” in Silicon Containing Polymers, Jones, et al., eds., Netherlands: Kluwer Academic Publishers, 2000, pp 185-212). Removal of the methyl group frees the silicon atom to react with silanol and siloxane groups on the silica surface, and the polymer also adsorbs non-covalently to the particle (see U.S. Pat. No. 6,503,676). U.S. Pat. No. 6,899,951 discloses conditioning mixtures of organopolysiloxanes with precipitated silicas; the polymer attaches to the silica surface via the siloxane bridges of the polymer.
Recently, it has been shown that colloidal silicas can be treated with small molecule agents, e.g., hexamethyldisilazane and alkoxysilanes, in aqueous media (see U.S. Patent Publications Nos. 2006/0171872 and 2008/0070140). Siloxanes may be employed to increase the hydrophobicity of the surface. For example, U.S. Pat. Nos. 6,830,811 and 6,184,408 disclose treatment of silica particles with short dimethylsiloxane oligomers. The reactions are carried out under acidic conditions under which the oligomers are cleaved. Thus, siloxane groups are attached to the surface.
However, it is desirable to treat toner additives with higher molecular weight silicones to provide additional hydrophobicity and reduce surface energy. It is further desirable to provide a surface treatment that will be stable with respect to organic solvents and polymers that can extract adsorbed hydrophobic compounds from the surface of silica toner additives. It is further desirable to provide a surface treatment for silica particles that reduces water adsorption.